It is known in offset printing to use cylinders lined with a printing blanket to permit the printing of a paper web which is pinched and driven between cylinders. Previously, the blankets were fastened onto the cylinders with their ends entered and locked into a longitudinally extending gap in the cylinder. This caused a number of inconveniences. In effect, the confronting ends of the blanket necessarily left a certain space therebetween, so that the paper web exhibited unprinted areas. Moreover, this way of fastening blankets into "gapped" cylinders imparted to the cylinder-blanket assembly a dissymmetry which generated vibrations during the rotation of the cylinder. Therefore, the speed and the efficiency of the printing machines was necessarily limited.
Gapped cylinders created a problem known as "fall off at the gap" for printing blankets having a fabric layer located between a printing surface and compressible foam layer. The fabric compressed the foam near the gap because it could not elongate sufficiently, and consequently decreased printing sharpness. U.S. Pat. Nos. 4,303,721 and 4,812,357 disclosed the use of an elastomer between the printing and foam layers to avoid fall off at the gap.
It is known that "seamless" and resiliently compressible blankets can be mounted around gapless cylinders in the manner of a continuous tube or sleeve.
For example, U.S. Pat. Nos. 3,983,287 and 4,378,622 disclosed tubular outer layers disposed around an inner compressible layer. The Canadian Patent Application No. 2,026,954 of Gaffney et al. suggested that a compressible foam layer disposed directly beneath a printing surface layer was needed to avoid bulges on either side of nip during operation, although it was also suggested that fabric could be inserted between layers.
U.S. patent application Ser. No. 07/682,048 of Bresson, filed Apr. 8, 1991, on the other hand disclosed a seamless blanket in which at least one hard elastomer layer, e.g. a substantially non-compressible material such as cured rubber, was employed between a surface printing layer and a compressible layer to minimize vibration in the blanket at high rotational velocities. The elastomer could optionally be reinforced with fibers. The multi-layered blanket was seamless in that it could be mounted around a cylinder without any surface interruptions, in the manner of a sleeve, thus permitting axial symmetry and allowing printing machines using such cylinders to operate at high speeds with minimum vibration.
Because seamless blankets are not secured by gaps in the cylinder, new problems arise regarding blanket installation and mounting, the avoidance of creeping or slippage during rotation, and removal after use, to name but a few. Unitary, cylindrically-shaped blankets can be axially mounted or dismounted on cylinders using compressed air, which is passed in a substantially radial direction from holes located within the cylinder. For example, U.S. Pat. No. 4,903,597 of Hoage et al. teaches that compressed air or gas is used to expand the sleeve to a limited extent for facilitating mounting and dismounting operations.
Thus, seamless blankets must be sufficiently resilient to provide compressibility for generating nip pressure; and yet they must have sufficient dynamic stability such that the circumferential (e.g. angular) velocity of the surface printing layer is not altered in passing through the nip. The uniformity of the velocity at which the printing surface passes through the nip is important to achieving web control (i.e. the printed material is not slipping relative to the rotating blanket) as well as to achieving good image resolution during rotation (i.e. no smearing of the image or distortion in the blanket surface).
Such antagonistic demands require a novel seamless, multi-layered printing blanket and method for making the same.